Polyamide resins have high resistance to heat, chemicals, and weathering and exhibits, for example, excellent mechanical and electrical properties. Polyamide resins are thus used in various industrial applications, such as injection molding materials, fibers, and films.
However, polyamide resins are highly crystalline. When they are formed into a thin product, a complex-shaped product having nonuniform thickness, or the like by injection molding, the product suffers from problems such as warpage resulting in deformation of the product. In order to overcome such problems, in general, incorporation of various inorganic particles has been attempted for improvement. However, this approach also has a problem such as low surface appearance of the product or orientation of fibrous inorganic substances resulting in anisotropy and thus deformation of the product.
Such problems of inorganic particles are presumably caused by insufficient dispersibility of inorganic particles or excessive size of dispersed particles. A technique for finely dispersing inorganic particles has been desired.
Japanese Unexamined Patent Application Publication Nos. 62-74957, 2-69562, 6-80873, 6-228435, 11-349811, 6-248176, 8-283567, 9-241505, and 2001-2913 disclose polyamide resin compositions containing swelling clay compounds, i.e., inorganic particles, finely dispersed in polyamide resins. In these techniques, swelling clay compounds, such as montmorillonite, are added during the polymerization for polyamide resins. However, clay compounds increase the melt viscosity of the polymer, thereby causing agitation failure during the polymerization. Thus, the amounts of clay compounds that can be used in the polymerization process are as small as approximately several percent, and this has limited the scope of the product design. Moreover, in order to compound auxiliary materials such as a flame retarder and a stabilizer, additional steps are necessary or the process becomes complicated. An improvement as to these points is also desired.
Another problem of the above-described techniques is the use of organic ammonium salts as a surface treatment agent for achieving homogeneous, fine dispersion of swelling clay compounds. Organic ammonium salts retained at a temperature for processing polyamide resins for a long time undergo deterioration, thereby degrading the properties such as mechanical properties or toughness. This problem must also be overcome.
Japanese Unexamined Patent Application Publication Nos. 8-319417, 2000-212432, 2000-290500, and 2001-302845 and International Publication No. 97-11998 disclose polyamide resin compositions containing swelling clay compounds finely dispersed in polyamide resins by extrusion. However, in these inventions, dispersion is insufficient, and thus the properties are not sufficiently improved. Moreover, the use of organic ammonium salts as a surface treatment agent for clay compounds may lead to deterioration during the processing and may degrade the properties such as mechanical properties or toughness. Among the above-described inventions, the invention set forth in Japanese Unexamined Patent Application Publication No. 2000-212432 requiring end-capped nylon and the invention set forth in International Publication No. 97-11998 requiring organic ammonium salts having bicyclo rings are not suitable for industrial applications.
Japanese Unexamined Patent Application Publication No. 9-118518 discloses a technique for facilitating fine dispersion by rendering layers of swelling clay compounds readily cleavable. In this technique, a polymeric compound (intercalant polymer), such as polyvinylpyrrolidone, is intercalated between layers of sheet silicate to prepare an intercalation compound. However, although this invention provides the intercalation compound, it does not provide a technique of cleaving the intercalation compound and finely dispersing the cleaved intercalation compound into a polyamide resin. It has been difficult to finely disperse a swelling clay compound into a polyamide resin.
Japanese Unexamined Patent Application Publication Nos. 10-259016 and 10-310420 disclose that, in order to cleave a layered swelling clay compound so that it can be finely dispersed in a thermoplastic resin, it is particularly effective to treat the swelling clay compound with a water-soluble compound to convert the compound into an intercalation clay compound. According to this technique, the elastic modulus and heat resistance can be improved without impairing surface appearance. However, further improvements on various properties and on warpage that occurs during injection molding are strongly desired.
As is apparent from the above, there has been no technique of homogeneously and finely dispersing a swelling clay compound in a polyamide resin by a simple process, such as melt-kneading, to obtain a polyamide resin composition having excellent properties.
In order to overcome problems arising from high crystallinity of polyamide resins, e.g., the problem of warpage and deformation that occurs in a thin product or a complex-shaped product with nonuniform thickness produced by injection molding, various approaches other than the use of inorganic particles have been attempted. Examples thereof include alloying of a polyamide resin with a noncrystalline resin, such as a polycarbonate resin, a styrene resin, or a polyphenylene resin. However, this results in degradation of surface appearance or heat resistance of the product, or in deformation of the product due to the anisotropy generated by orientation of fibrous inorganic substances.
An example of alloying a polyamide resin with a styrene resin is disclosed in Japanese Examined Patent Application Publication No. 38-23476 in which a polyamide resin is alloyed with an ABS resin to prepare a polyamide/ABS alloy. Examples of the method for improving the compatibility between a polyamide resin and an ABS resin are disclosed in Japanese Unexamined Patent Application Publication Nos. 63-179957 and 64-158, in which a modified copolymer prepared by copolymerization of styrene and acrylonitrile with an unsaturated carboxylic acid is blended. The alloying method using an inorganic filler and a particular type of styrene resin is disclosed in Japanese Unexamined Patent Application Publication Nos. 4-120167, 4-332758, 8-143768, and 9-217006. The method using a combination of ABS and polyamide prepared by polymerization in the presence of lamellar silicate is disclosed in Japanese Unexamined Patent Application Publication No. 8-3439. The method using a combination of talc, a particular styrenic compound, and a polyamide containing finely dispersed lamellar silicate is disclosed in Japanese Unexamined Patent Application Publication No. 2000-212431. The method using a combination of a thermoplastic elastomer, a styrenic rigid polymer, and a polyamide containing a swelling fluorine mica-type mineral is disclosed in Japanese Unexamined Patent Application Publication No. 9-12873.
However, none of these techniques can simultaneously yield satisfactory surface appearance, deformation properties, heat resistance, and mechanical properties. For example, according to these techniques, the surface appearance (surface quality and low sink-mark formation) is not sufficient, the mechanical properties or thermal properties are degraded by water absorption, or heat resistance is low.
As is described above, polyamide resins are used in various fields. However, polyamide resins have high water absorption and thus tend to exhibit degraded properties once they absorb water. An approach for reducing the water absorption known in the art is to blend polyamide resins with less water-absorbing resins, such as polyolefin resins (Japanese Unexamined Patent Application Publication Nos. 05-043794 and 06-136259; Fumio IDE, Kazumasa KAMATA, Akira HASEGAWA, “Kobunshi Kagaku [Macromolecular chemistry]” The Society of Polymer Science, Japan, Feb. 25, 1968, vol. 25, No. 274, pp. 107-115). However, the polymer blending sometimes results in low heat resistance or lower stiffness. An approach for maintaining the stiffness known in the art is to add fibrous reinforcing materials or inorganic fillers (Japanese Unexamined Patent Application Publication Nos. 06-200087, 06-234896, and 07-108619). However, addition of fiber reinforcing materials results in warpage or deformation of the product due to anisotropy, and addition of inorganic fillers results in low surface quality. An approach for maintaining the surface quality is to add swelling silicate treated with ammonium salts (Japanese Unexamined Patent Application Publication Nos. 10-279752 and 11-181277). However, ammonium salts have low resistance to heat and thus undergo heat deterioration when subjected to heating for a long time at a temperature of processing polyamide resins, thereby resulting in deficiencies such as coloring.
As described above, there has been no technique of producing polyamide resin materials that show satisfactory surface quality, low warpage, high stiffness, and low water absorption without causing heat deterioration, such as coloring, during the melt processing of polyamide resins.
Recent development in electronic technology has led to an increase in demand for electrostatic recording sheets; housings for electronic devices; electrostatic containers; electrostatic films; floor covering, wall materials, and partitions for clean rooms; covering materials for electronic devices; and magazines for ICs and the like. The materials used therefor must satisfy the required dimensional accuracy, low warpage, surface quality, high stiffness, and heat resistance. Furthermore, since polyamide resins have excellent heat resistance, mechanical properties, and the like, they are used in many industrial applications, such as materials for injection molding, sheets, and films. However, polyamide resins readily become electrostatically charged and thus cannot be directly applied to films, sheets, and containers composed of boards because accumulation of electrostatic charge should be avoided. Accordingly, a technique of rendering electric conductivity to synthetic resins is widely employed. In general, for example, carbon fibers (Japanese Unexamined Patent Application Publication Nos. 7-205310 and 10-237316) and carbon black (Japanese Unexamined Patent Application Publication Nos. 11-310701 and 7-331029) are used.
However, when products are fabricated using materials containing carbon fibers for rendering conductivity, carbon fibers frequently appear in the surfaces of the products. Thus, when such products are used as containers or transfer trays, the surfaced carbon fibers will damage ICs and other electronic components. Furthermore, during molding, carbon fibers align in the direction of flow, thereby generating anisotropy. This leads to another problem, i.e., warpage in the molded products. When carbon black is used, inorganic fillers are used to provide stiffness and heat resistance; however, inorganic fillers decrease surface quality and cause warpage.
As is stated above, a technique that can provide conductive polyamide resin compositions having excellent heat resistance, stiffness, and surface quality while achieving low warpage has not been found so far.